Inductive Charging Takes Shape at BMW & Daimler

BMW has shared some insight into its development of inductive charging schemes for electric vehicles. In the medium term, the company plans to launch series production for the technology. The project is conducted along with competitor Daimler. Each company plans to provide a uniform charging technology for the garage at home.

The system consists of two components: A primary coil integrated into a base plate that is placed beneath the vehicle, for instance in the floor of a garage or parking lot. This coil induces electric energy to the secondary coil in the car floor.

The arrangement of the coils, and consequently of the field pattern, is based on a design derived from their circular shape that offers a number of benefits such as a compact yet light construction as well as an effective spatial confinement of the magnetic field -- a feature important to maintain high efficiency. The alternating magnetic field between the coils transmits the electric energy wirelessly at a power of up to 3.6 kW. BMW specifies the energy efficiency of this arrangement at 90%. The system aims at charging high-voltage batteries for plug-in hybrid and battery electric cars.

One of the design goals was achieving the shortest possible charging time. At a charging power of 3.6 kW, many plug-in hybrids can be charged completely in less than three hours. The BMW i8, the company's current technology flagship, can be charged in less than two hours. The i8 has a plug-in hybrid powertrain, while its smaller sibling i3 is a battery electric car. To take higher battery capacities for purely electric vehicles into account, the future wireless charging technology standard offers the option to increase the charging power to 7kW, which would make it possible to charge the batteries of the i3 over night.

To charge up a vehicle with the system, the car needs to be positioned exactly over the primary coil. A software in the head unit and a WiFi data link between charging station and vehicle supports the driver during this task. In addition, a smartphone app allows users to monitor the charging progress and informs about the remaining charging time.

This is how the components for wireless charging are arranged in BMW's system (Stromversorgung = power supply; Sekundärspule = secondary coil; Gleichrichter = rectifier; Energiespeicher = energy storage).

The system is weather-independent; it also works if the surface is covered with snow or water. The electric stray field is reduced to the minimum possible, says BMW -- without, however, specifying how strong the field is. The gap between primary and secondary coils is monitored constantly; if any foreign particle enters the gap, the charging process is stopped immediately, potentially saving the live of any curious pet cat.

>> If I'm paying $100k for a car then I don't care about an extra 10% on energy and $10k to install an inductive coil in my garage just to save the effort to plug it in every day.

In reality, we are not paying the real value. In economics, they have inelastic and elastic demand. There are things that have value because few can afford them. In you buy a car for $100k because you want to save on gas, that person has a problem with math!

Inductively charging a car in the garage at home (with suitable positioning information) adds a level of convenience but I see serious issues associated with charging elsewhere. Not only will the industry need to standardize on the system used in your car (which you may keep for 8 or more years) but also the neighboring cars will need to park properly so that your car can be precisely parked over the inductive coil. Will drivers have to learn the art of precision parallel parking - or will charging spots be well separated diagonal / perpendicular parking places that are easy to access?

I'm a bit dubious of the 90% figure. I wonder if you could really get that efficiency in the field. It just seems like it might be better to have an automated system that engages a mechanical connection, perhaps also from underneath the car.

Agreed. That's a huge loss of electricity/money over the lifetime of a car. I think a robotic arm that physically connects the two would be much more practical. No need for precise driving, much less energy losses, and if designed right, would still allow quick disconnects happening as soon as you open the car door.

I don't have any doubt about the technical feasibility of this. The technology behind this isn't "rocket science." Practicality is very different than technical feasibility or even production worthiness.

I just didn't see the value will justify the costs. Some of the points made by other posters are good though. If I'm paying $100k for a car then I don't care about an extra 10% on energy and $10k to install an inductive coil in my garage just to save the effort to plug it in every day.

Let us say "not very practical, YET". I like to remind people that before SpaceX and Tesla, there were many things people said were not possible in the industries. But Elon Musk and co have just done them so effectively that Detroit and others are worried. Time will tell if this can be done in a practical-production scale.

It is only impractical if your only goal is efficiency. If your goal is convenience, then it is very practical. Electric cars (especially from Tesla, BMW, and Daimler) are very expensive, which means they are luxury cars. Being able to just get in your car and drive without fiddling with power cords is an added luxury. I'm guessing that the extra 10% electricity cost isn't an overriding issue for such customers.

Lots of people use inductive chargers for their cell phones and tablets, and it is equally inefficient. I personally don't mind plugging in the cable (for my phone), but I can understand that some people prefer the convenience of just throwing thier phone on the charging pad when they get home.